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Help: Diode doesn't behave as it should when connected to coil

  1. Jul 4, 2011 #1
    It's pretty straightforward. I built a circuit to energize a coil and connected a diode in parallel to make sure the discharge won't harm the transistor. The diode is in working order, I've tested it with a multimeter and found that it has a high resistor value when forward-biased and it doesn't conduct when reverse-biased.

    Now, if I connect the diode to the coil (and nothing else, I don't even connect the battery) and proceed to measure the resistance value across the diode, I find that the value is the same for forward-biased and reverse-biased and it's around 0.04 ohm (as if the diode were acting like a conductor). This would explain why I found no voltage differential on the terminals of the coil when I connected the battery, but I am at a loss to explain why this happens. I simulated the circuit using NI Multisim and it works like a charm.

    Can anybody tell me what I'm doing wrong?
  2. jcsd
  3. Jul 4, 2011 #2


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    If you connect the diode across the coil, you would be measuring the resistance of the coil, which could easily be less than one Ohm.

    Also, check if you meter has a "diode" setting. This will give you the forward voltage across the diode when the diode is not across the coil. This is a useful measurement as it tells you if the diode is OK and sometimes what type of diode it is. Silicon diodes will normally have a voltage drop of 0.6 to 0.7 volt when tested like this.

    Measuring the resistance of a diode using a digital multimeter will sometimes not work because these meters use a very low voltage to do the measurements.
  4. Jul 4, 2011 #3
    I think it does have a diode setting, I'll have to try it.

    I considered that maybe the multimeter was measuring the equivalent resistance of the set, but the coil has a resistance value of 3.4 ohm, so I still don't know where the 0.04 ohm come from.
  5. Jul 5, 2011 #4


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    The only way a diode can measure 0.04 ohms is if it is faulty.
    If it was faulty, it wouldn't repair itself, so it would still be faulty.

    Maybe you could try it again. Measure the coil resistance and while you are doing this, put the diode across it. See if you still get the same reading.

    Most digital multimeters won't measure 0.04 ohms, so maybe you were reading the scale wrongly?
  6. Jul 5, 2011 #5
    0.04 ohms may be possible if you read through the diode in reverse bias while it was in parallel with the coil of the relay. the total resistance of the two would be less than both of them seperately. 0.4 ohms would be more realistic. Other things to consider would be the quality of the meter and whether or not these two components were in circuit with anything else that would create another parallel conducting path. When I see readings this low in industrial electronics, its usually the result of several paths in parallel, sometimes with as little resistance as a simple trace.
  7. Jul 5, 2011 #6


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    It is unlikely that a good diode will even be forward biased when in parallel with a coil of wire with an ohmeter applied.
  8. Jul 5, 2011 #7
    I'll second this. This may be your DMM's way of telling you to change the scale.
  9. Jul 5, 2011 #8


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    I have seen, and used, meters that give readings as low as this, but they usually cost many $$$. This function usually requires a pot to balance out the resistance of the multimeter leads.

    Most meters have a 200 ohm scale for resistance and these give a minimum reading of 0.1 ohm.

    There are some that have a "2K" scale and can give a reading of "0.04" but it means 0.04 K or 40 ohms.

    Auto ranging meters are particularly deceptive and I never use one. They can change the scale by a factor of 1000 and you may not even notice it.
  10. Jul 5, 2011 #9
    Disconnect the diode from the coil. Measure it with a diode checking meter; those don't read read ohms, they read volts. A 0.4 volt drop one way and open circuit is a good diode. Low readings in both directions or open circuit is a dead diode.
  11. Jul 15, 2011 #10
    Apparently, it wasn't the diode that was shorted but the transistor itself. Tried with another one and it worked.
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